1. Field of the Invention
The present invention relates to an air-fuel ratio control apparatus for an engine and more particularly to an air-fuel ratio control apparatus for an engine comprising sensors, provided respectively on the upstream and downstream sides of a catalytic converter, for detecting an air-fuel ratio of exhaust gas passing through the catalytic converter to implement air-fuel ratio feedback control on the basis of the air-fuel ratio detected by the downstream sensor, in addition to air-fuel ratio feedback control on the basis of the air-fuel ratio detected by the upstream sensor.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. 61-232350 discloses an air-fuel ratio controller which guards against deviation from a centered control value when an oxygen (O.sub.2) sensor for detecting O.sub.2 concentration in exhaust gas on the upstream side of a three-component catalytic converter deteriorates.
Also, the air-fuel ratio controller disclosed in Japanese Patent Application Laid-Open No. 2-238147 controls an air-fuel ratio correction coefficient FAF based on an output voltage VOX2 of an O.sub.2 sensor on the downstream side of a catalytic converter as shown in FIG. 19. Using this system, the actual air-fuel ratio converges on a stoichiometric air-fuel ratio using O.sub.2 sensors respectively on the upstream side and downstream side of the catalytic converter to determine whether the exhaust gas is rich or lean based on the output voltage of the O.sub.2 sensor on the upstream side. This is done by driving the air-fuel ratio correction coefficient FAF in the opposite fluctuation direction as the air-fuel ratio using predetermined constants of integration KIR and KIL, and by driving the air-fuel ratio correction coefficient FAF to the opposite side of the fluctuation direction of the air-fuel ratio through a skip discontinuity by skip amounts RRS and RSL. Further, when the output voltage VOX2 of the O.sub.2 sensor on the downstream side is richer than a predetermined threshold value VRL or leaner than a predetermined threshold value VLL, the skip amounts RSR and RSL described above are increased to effect a large change in the air-fuel ratio correction coefficient FAF in order to complete the correction of the air-fuel ratio quickly.
Furthermore, using the system disclosed in Japanese Patent Application Laid-Open No. 3-185244, the actual air-fuel ratio converges on a stoichiometric air-fuel ratio by providing O.sub.2 sensors respectively on the upstream side and downstream side of the catalytic converter to determine whether the air-fuel ratio is rich or lean based on the output voltage VOX2 of the O.sub.2 sensor on the downstream side of the catalytic converter and by driving the target air-fuel ratio in the opposite fluctuation direction at a constant speed using a predetermined rich integration amount IR and a predetermined lean integration amount IL, as shown in FIG. 20. Then, a correction coefficient FAF is calculated at a predetermined updating speed based on a difference between the target air-fuel ratio after the correction and the actual air-fuel ratio detected by the O.sub.2 sensor on the upstream side of the catalytic converter.
The use of a sub-feedback (F/B) system employing guard values has been proposed. In such a system, the guard values prohibit excessive deviation from a central control value when the O.sub.2 sensor on the upstream side deteriorates.
Further, because the skip amounts RSR and RSL based on an output voltage VOX1 of the O.sub.2 sensor on the upstream side are increased and decreased based on the output voltage VOX2 of the O.sub.2 sensor on the downstream side as shown in FIG. 19, the correction amount effected by the O.sub.2 sensor on the downstream side is reflected in the air-fuel ratio correction coefficient FAF only when the air-fuel ratio detected by the O.sub.2 sensor on the upstream side crosses the stoichiometric air-fuel ratio and the skip amounts RSR and RSL are used. Accordingly, even if the O.sub.2 sensor on the downstream side detects that the air-fuel ratio has exceeded the rich side allowable value VAL at time t1, the air-fuel ratio correction coefficient FAF is corrected by the skip amount RSL which has increased based on the value detected at a considerably delayed time t2. Then, due to an overcorrection caused by the delay, the air-fuel ratio fluctuates periodically between the rich side and the lean side without converging on the stoichiometric air-fuel ratio and thereby CO, HC and NOx alternately appear in the exhaust gas.
Furthermore, because the air-fuel ratio correction coefficient FAF is calculated at a predetermined updating rate based on the difference between the target air-fuel ratio after correction by the output voltage VOX2 of the O.sub.2 sensor on the downstream side and an actual air-fuel ratio as indicated by the output voltage VOX1 detected by the O.sub.2 sensor on the upstream side as shown in FIG. 20, the rich integration amount IR and lean integration amount IL are immediately reflected in the air-fuel ratio correction coefficient FAF. However, since the engine, including the catalytic converter, is a system originally having a large delay, the air-fuel ratio on the upstream side is already largely disturbed in either direction from the stoichiometric air-fuel ratio at the point in time when the fluctuation direction of the air-fuel ratio of the exhaust gas has been inverted between rich and lean, and at that point it is difficult to suppress the turbulence of the air-fuel ratio caused on the downstream side thereafter by the delicate correction by means of the rich integration amount IR or lean integration amount IL. Accordingly, there has been a problem similar to the case described above, where the air-fuel ratio is overcorrected due to the delay in correction and does not converge to the stoichiometric air-fuel ratio, and that CO, HC or NOx are discharged from the engine.